Treatment resistant depression is a chronic, disabling and life-threatening disease that affects as many as 30% of individuals diagnosed with major depressive disorder. For these patients, traditional pharmacological treat- ments are often not effective, and deep brain stimulation of discrete brain regions has emerged as one of the only viable treatment options. One target area for such treatment is the lateral habenula (LHb), a component of the epithalamus that receives confluent input from emotional and motor systems, strongly influences activity of midbrain dopamine (DA) neurons, and is increasingly implicated in aversive stimulus processing, learning, and clinical depression. However, the circuitry of this system and the critical role of newly discovered components within the system remain incompletely understood. For example, the role of the newly identified rostromedial tegmental nucleus (RMTg), a GABAergic midbrain region which receives LHb input and projects intensely to DA neurons, is largely unexplored, although it's known properties suggest possibly central roles in depressive phenomena. Our proposal addresses several fundamental outstanding questions regarding these habenulo- mesencephalic circuits using a range of behavioral and electrophysiological techniques that are grouped into three related aims. In Aim 1, we will test the hypothesis that the physiological and behavioral effects associated with acute and chronic activation of the LHb are mediated via a projection from the LHb to the RMTg. Aim 2 will test the hypothesis that the LHb and RMTg critically contribute to maladaptive behaviors in two distinct animal preparations that model human depression endophenotypes. This aim uses lesion and stimulation studies to characterize the importance of this pathway in depression-related responses to both aversive and rewarding stimuli (despair, anhedonia). Finally, in Aim 3, we will test the hypothesis that depression-induced changes in LHb and RMTg patterns of firing underlie the behavioral and cognitive deficits seen in depressive disorders. Overall, these studies investigate the proposition that dysregulation of a fundamental circuit, LHb-RMTg-VTA, is involved in the maladaptive response to adverse events. Results obtained from these experiments will ad- dress a major focus of the NIMH Research Domain Criteria (RDoC) efforts to define mental disorders based on neurobiological measures that cross diagnostic boundaries. Dysfunction of brain reward systems has clear im- plications for depression, and also schizophrenia, bipolar disorder, and drug abuse. Given growing convergent preclinical and clinical data, these studies have clear translational relevance. PUBLIC HEALTH RELEVANCE: Connections between the habenula and midbrain dopamine neurons are part of a circuit involved in learning from environmental cues that predict undesirable outcomes such as aversive stimuli or the omission of an expected reward. In this application, we propose experiments designed to identify a missing component in this pathway and test the hypothesis that repeated activation of this circuit contributes to the development of depressed behavior. In addition to providing new insights into the underlying pathophysiology of an illness that affects 15 million American adults annually, this program of research will address a major focus of the NIMH Research Domain Criteria (RDoC) efforts to define mental disorders based on neurobiological measures that cross diagnostic boundaries and advance our understanding of the cognitive and emotional changes that occur in patients with major depressive disorder.